Cemented carbide body with high wear resistance and extra tough behavior

ABSTRACT

There is provided an Al2O3-TiN coated cemented carbide insert intended for turning of steels and especially Ca-treated steels. The alumina layer is protected by an extra thick and multilayered coating of TiN. The TiN coating is wet blasted and, for this purpose, (Ti,Al)(C,O,N) bonded to the underlying Al2O3 layer which preferably consists of  alpha -Al2O3. The extra tough behavior together with increased wear resistance can be obtained by optimizing the TiN layer thickness, structure and adhesion and wet blasting the said TiN coating. This invention is characterized by the fact that the coating layers are not missing at the cutting edge of the insert.

BACKGROUND OF THE INVENTION

Aluminum oxide coated cemented carbide cutting tools have been inindustrial practice for well over 15 years and are today commonly usedfor the turning and milling of cast irons and steels.

Al₂ O₃ coatings are usually deposited by chemical vapor deposition (CVD)after applying an intermediate layer of TiC, TiN or Ti(C,N), either as asingle layer or as multilayers onto the cemented carbide substrate.Instead of using intermediate layers, the cemented carbide substrate canbe enriched with γ-carbide phase, i.e., a solid solution of cubiccarbides of titanium, tantalum, niobium and tungsten, prior to Al₂ O₃deposition. In order to enhance adhesion of alumina to the intermediatelayer, one or several interlayers, usually referred to as bondinglayers, can be applied between the intermediate layer and the aluminalayer. Furthermore, the alumina coating itself may be deposited asmultilayers. Accordingly, a plurality of coating combinations exist.

The CVD alumina layers are usually composed of pure κ-Al₂ O₃, mixturesof κ-Al₂ O₃ and α-Al₂ O₃ or pure α-Al₂ O₃. A layer of TiN is usuallydeposited on top to the alumina layer. The TiN layer is usuallyrelatively thin (1 to 2 μm) and is mainly used to give the tool anattractive appearance--the so-called "golden color". For example, theTiN layer, which is referred to as a finish layer in U.S. Pat. No.4,984,940, is used to provide a low friction surface and to minimizemetal build-up on the coating. Accordingly, the TiN in this case isrelatively thin, about or less than 2 μm and much less than 4 μm,although it is disclosed in this patent to be 0.2-4 μm.

In addition to the golden colored alumina coated inserts, black inserts,i.e., alumina coating without the TiN top coat, are present in themarketplace and have, in fact, gained great industrial success. Theseinserts are often wet blasted in order to enhance the chippingresistance and surface finish. It has earlier been assumed that wetblasting must be performed directly on the alumina coating for two mainreasons:

1) The TiN layer deposited on top of the alumina layer is assumed todeteriorate the wet blasting effect; and

2) Wet blasting of TiN coated alumina inserts cannot be controlled inproduction scale due to the poor adhesion between TiN and alumina. Forthis reason, there have not been any wet blasted TiN coated or Al₂ O₃-TiN coated inserts on the market.

Consequently, the TiN coated alumina inserts are usually brushed inorder to enhance the chipping resistance and edge strength. Brushing ofthe TiN coated (golden colored) alumina inserts results in a black linealong the cutting edge (area where the TiN layer is worn away).

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to avoid or alleviate the problems ofthe prior art.

It is further an object of this invention to provide a Al₂ O₃ coatedcemented carbide cutting tool with high wear resistance and extra toughbehavior.

In one aspect of the invention there is provided a coated cementedcarbide body comprising a substrate of a cemented carbide, a layer ofAl₂ O₃ and an outer layer of wet blasted TiN at least 4 μm in thickness.

In another aspect of the invention there is provided a method of cuttingCa-treated steels using a cemented carbide cutting insert, theimprovement comprising using a cutting insert of a coated cementedcarbide body comprising a substrate of a cemented carbide, a layer ofAl₂ O₃ and an outer layer of wet blasted TiN at least 4 μm in thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a representation of one embodiment of the invention relatingto the deposition of the alumina layer as described below.

FIG. 1b is a representation of a second embodiment of the inventionrelating to the deposition of the alumina layer as described below.

FIG. 1c is a representation of a third embodiment of the inventionrelating to the deposition of the alumina layer as described below.

FIG. 2 is a graph of flank wear of an insert used to cut medium carbonsteel.

FIG. 3 is a graph of flank wear of an insert used to cut cast iron.

FIG. 4 is a graph of relative wear resistance vs. TiN layer thickness intests involving different materials.

FIG. 5 is a graph of the number of passes obtained with certain insertseither as coated or wet blasted.

FIGS. 6a-6c are representations of typical SEM's (scanning electronmicroscopy) of certain inserts showing their response to blasting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

It has been found that a thick (≧4 μm) layer of TiN deposited on top ofthe alumina layer improves the cutting performance of the insert byseveral hundred percent over prior art products in steels. An especiallypronounced effect can be obtained in Ca-treated steels. Flank wearresistance can especially be improved.

It has also been found that wet blasting can be performed on the TiNlayer without sacrificing the chipping resistance. For this purpose, theadhesion of the TiN layer has to be enhanced and the TiN layer itselfhas to be multilayered.

According to the present invention, a multilayered, coated cementedcarbide is provided. As shown in FIGS. 1a, 1b and 1c, the coating iscomposed of three main layers:

1) an inner layer 10 of Ti(C,N) with a thickness of at least 4 μm;

2) an intermediate layer 12 of Al₂ O₃ with a thickness of at least 6 μm;and

3) an outer layer 14 of a TiN/TiC multilayer with a thickness of atleast 4 μm, preferably greater than 4 μm.

This TiN/TiC multilayer is characterized as a relatively thickmultilayer of mainly TiN applied on top of a thick alumina layer. It hasbeen found to be important to combine the positive effects of TiN andAl₂ O₃ in this way, i.e., the alumina layer is not alone considered asan "active" layer and the TiN layer is not deposited as a "finish"layer.

As shown in the Example below, this product shows a superior performancein the cutting of steels compared with prior art products. The TiN/TiCmultilayer is especially important when Ca-treated steels are thematerial to be cut. The alumina layer underneath the TiN/TiC multilayeris also important as a thermal barrier reducing plastic deformation athigher cutting speeds.

The inner layer 10 is deposited directly on the cemented carbidesubstrate 8, the inner layer being at least one layer of TiC, Ti(C,N),TiN, Ti(C,O) or Ti(C,O,N). This layer is preferably Ti(C,O,N) andTi(C,N) with thin intermediate layers of TiC or TiN being inserted inthis layer in order to prevent transport of cobalt to the Ti(C,N)-Al₂ O₃interface. The thickness of this layer is in the range of 4 to 10 μm,preferably from 5 to 6.5 μm.

The outer layer 14 is a multilayered TiN-TiC coating deposited on top ofthe intermediate layer 12 of aluminum oxide. This layer 14 must bemultilayered in order to obtain grain refinement via renucleation. Witha single TiN layer, relatively large TiN grains would otherwise developresulting in poor wet blasting properties.

This outer layer is composed of 5-20 layers, preferably 6-10 layers, ofTiN separated by thin, <0.01 μm, layers of Ti(C,N) or TiC, preferably ofTiC. It has further been found to be advantageous to dope the TiN layerswith carbon. For example, ≦1% CHy can be added to the reactive gassesduring deposition of every second layer of TiN. Increased growth rateand further grain reinforcement can be obtained (FIG. 6).

The total thickness of the TiN multilayer is in the range of 4 to 15 μm,preferably in the range of 4 to 6 μm. The first multilayer canpreferably be pure TiC instead of TiN when deposited on a (Ti,Al)(C,O,N)bonding layer on α-Al₂ O₃.

The intermediate layer is aluminum oxide. This alumina layer must beeither pure (100%) α-Al₂ O₃ or pure (100%) κ-Al₂ O₃. The purity of thephase of the alumina is important because the K-C phase transformationduring deposition of the TiN multilayer would result in high stresses atthe Al₂ O₃ -TiN interface and would consequently weaken the adhesion ofthe TiN layer. The thickness of the alumina layer should be in the rangeof 6 to 20 μm, preferably in the range of 8 to 12 μm. The alumina layershould preferably be composed of α-Al₂ O₃ due to its better performancein cast iron as disclosed in U.S. Pat. No. 5,137,774.

It has however, been found that TiN, TiC and Ti(C,N) exhibit a pooradhesion when deposited directly on an α-Al₂ O₃ layer and good adhesionwhen deposited directly on a κ-Al₂ O₃ layer 18. Good adhesion between,for example, TiN and κ-Al₂ O₃ is assumed to be due to epitaxy at thisinterface. For this reason, a special bonding layer 16 should be usedwhen TiN or TiC is deposited on α-Al₂ O₃. The α-Al₂ O₃ can also bemodified according to U.S. Pat. No. 5,137,774. Consequently, there arethree main alternatives to deposit the alumina layer:

1) α-Al₂ O₃ +[(Ti,Al)(C,O,N)/TiC]-bonding layer+multi TiN/TiC. (See FIG.1a) The α-Al₂ O₃ is deposited as described in U.S. Pat. No. 5,137,774 orobtained as described in U.S. Pat. No. 5,635,247.

2) α-Al₂ O₃ +(Ti,Al)(C,O) modification layer+κ-Al₂ O₃ +multi TiN/TiC.(See FIG. 1b) The α/κ multioxide is deposited according to U.S. Pat. No.5,137,774. The κ-Al₂ O₃ on α-Al₂ O₃ should be in the range of 0.5-1.5μm. A multilayer TiN/TiC coating can be deposited with sufficientadhesion onto this κ-Al₂ O₃ layer.

3) κ-Al₂ O₃ +multi TiN/TiC. (See FIG. 1c) The κ-Al₂ O₃ layer can bedeposited as described in U.S. Pat. No. 5,137,774 or should preferablybe composed of multilayers (8×κ-Al₂ O₃) as described in U.S. Pat. No.5,635,247. No bonding layer is needed.

The relatively long heat-treatment during deposition of the TiNmultilayer may cause the κ-α transformation to occur. For this reason,alternative number 1 is preferred.

Wet blasting the outer surface 20 of the outer layer 14 was carried outwith Al₂ O₃ particles (320 mesh, medium grain size ⁻ 30 μm) in watersuspension. The applied pressure was 2-6 bar, preferably 3 bar. Theinserts were rotated while wet blasted. Both sides of the inserts werewet blasted.

The invention is additionally illustrated in connection with thefollowing Example which is to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Example.

EXAMPLE

The coating combinations described below are CVD deposited on commercialCNMG 120408-M3 cemented carbide inserts. The coating combinations areused to demonstrate the following:

the influence of the TiN layer deposited on top of the alumina layer forcutting performance in steel and cast iron (Series A, Table 1);

the influence of the Al₂ O₃ /TiN thickness ratio for the cuttingperformance in steel and cast iron (Series B, Table 2);

optimization of the toughness of these inserts using wet blasting; and

optimization of the blasting response of the coated inserts (adhesionand the mechanical properties of the TiN layers, Series C, Table 3).

The cutting test results are shown for steel and cast iron in Tables 4and 5, respectively. The failure mode of the TiN coated inserts isgenerally flank wear in steel and cast iron (FIGS. 2 and 3). Aluminalayers without top TiN typically exhibited crater wear. As is clear fromFIG. 4, the performance (lifetime) is strongly increased as a functionof the TiN layer thickness up to about 8 μm in steel, and especially inCa-treated steels. However, the cutting performance is found todeteriorate in cast iron when the TiN layer thickness exceeds 5 μm.

As the coating thickness increases, residual stresses are built up inthe coatings. The residual stresses reduce the strength of the coatingsand can be often seen as increased chipping tendency (brittleness) ofthe coated tools. The residual stresses can be decreased by wet blastingof the coated tools. The wet blasting reduces the chipping tendencydrastically.

As shown in Table 6 and FIG. 5, the chipping resistance of the as-coatedinserts is not sufficient. The wet blasting increases the chippingresistance of the TiN coated alumina coatings drastically, but againonly to a certain TiN layer thickness. This is about 5 μm. Consequently,the thickness of the TiN layer must be limited to this thickness (about5 μm) in order to obtain sufficient toughness (the 40 passes in thelaboratory conditions have been found to correlate with sufficienttoughness in the machine shops).

The alumina layer is important under the TiN layer when higher cuttingspeeds are concerned (Table 7). Although I do not wish to be bound byany specific theory, it is speculated that the alumina layer acts as athermal barrier and consequently reduces plastic deformation of theinserts. At cutting speeds under 200 m/min, one could expect that thealumina coated inserts would perform no better than TiN coated inserts.

As pointed out above, blasting is of utmost importance when toughness isconcerned. In production scale, it is impossible to obtain acceptableresults with a single TiN layer (FIG. 6a). Better results are obtainedwith multi TiN layers (FIG. 6b) or a single TiN layer in combinationwith a (Ti,Al)(C,O,N) bonding layer. The only acceptable solution,however, is a multilayered TiN coating which is bonded to a Al₂ O₃ layervia a (Ti,Al)(C,O,N) bonding layer, Table 8, FIG. 6c. It is emphasizedhere that the coating layers are not missing at the cutting edge as, forexample, is the case with the prior art products (when TiN overlays Al₂O₃ )(see, for example, U.S. Pat. No. 5,597,272). It is also noted thatthe cutting performance in steel was slightly increased when the TiNlayer was multilayered and properly bonded to the alumina layer. In castiron, failures through chipping and flaking at the coatings occurredwhen the thickness of the TiN layer exceeded 5 μm.

When machining modern Ca-treated steels, the cutting speeds can besubstantially increased. However, the coating materials do not possessthe same properties in these steels as compared to conventional steels.For example, Al₂ O₃ is not necessarily the most stable coating material.It has been suggested that the elements present in the Ca-treated steelsreact with alumina to form a liquid with low melting temperature. Thisliquid is easily removed by the chip resulting in a large wear rate.

The present tests show that TiN and TiC coatings are much more effectivewith respect to crater wear resistance and flank wear resistance inCa-treated steels than Al₂ O₃ coatings. However, Al₂ O₃ coatings aremuch more effective than TiC and TiN coatings with respect to notch wearresistance and obviously Al₂ O₃ may act as a thermal barrier making thehigher cutting speed possible. These tests also show that the mosteffective way to combine these positive effects to TiN/TiC and Al₂ O₃coatings is to deposit them as multilayers so that the TiN/TiC coatingoverlays the alumina coating (as shown above for conventional steel). Anaverage increase in lifetime in Ca-treated steel is 50%-100% better thanthat obtained in conventional steels over the prior art products (FIG.4).

                  TABLE 1                                                         ______________________________________                                        Coating Structures From Substrate Outwards (Series A)                         ______________________________________                                        No. 1      Ti(C, N)    6 μm                                                           α-Al.sub.2 O.sub.3                                                                  8 μm                                                No. 2      Ti(C, N)    6 μm                                                           α-Al.sub.2 O.sub.3                                                                  8 μm                                                           TiN         3 μm                                                No. 3      Ti(C, N)    6 μm                                                           α-Al.sub.2 O.sub.3                                                                  8 μm                                                           TiN         5 μm Σ19 μm                                No. 4      Ti(C, N)    6 μm                                                           α-Al.sub.2 O.sub.3                                                                  8 μm                                                           TiN         9 μm                                                No. 5      Ti(C, N)    6 μm                                                           α-Al.sub.2 O.sub.3                                                                  8 μm                                                           TiN         11 μm                                               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Coating Structures From Substrate Outwards (Series B)                         ______________________________________                                        No. 6      Ti(C, N)    6 μm                                                           Al.sub.2 O.sub.3                                                                          0 μm                                                           TiN         13 μm                                                                              Σ19 μm                                No. 7      Ti(C, N)    6 μm                                                           Al.sub.2 O.sub.3                                                                          4 μm                                                           TiN         9 μm Σ19 μm                                ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Coating Structures (Series C)                                                 (Used To Test Wet Blasting Response)                                          ______________________________________                                        No. 8    Ti(C, N)   6 μm                                                            α-Al.sub.2 O.sub.3                                                                 8 μm                                                            TiN        5 μm (multi, composed of 6 layers of                                       TiN separated by TiC layers)                              No. 9    Ti(C, N)   6 μm                                                            α-Al.sub.2 O.sub.3                                                                 8 μm                                                            bonding    (Ti, Al)(C, O, N) ≦ 0.5 μm                               TiN        5 μm (single)                                          No. 10   Ti(C, N)   6 μm                                                            α-Al.sub.2 O.sub.3                                                                 8 μm                                                            bonding    (Ti, Al)(C, O, N) ≦ 0.5 μm                               TiN        5 μm (multi, as No. 8)                                 ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Tool Life and Failure Mode, Steel SS1672                                      (As-coated and Wet Blasted)                                                   ______________________________________                                        Turning:             SS1672                                                   Cutting Speed:       350 m/min.sup.-1                                         Feed:                0.25 mm/r.sup.-1                                         Depth of Cut:        2.5 mm                                                   Coolant:             no                                                       ______________________________________                                        Coating    Failure Mode Average Lifetime/mm*)                                 ______________________________________                                        No. 1      crater wear  4                                                                flank wear                                                         No. 2      flank wear     7.5                                                 No. 3      flank wear**)                                                                              11                                                    Nos. 4 and 5                                                                             flank wear**)***)                                                                          12                                                    ______________________________________                                         *)note: Average Lifetime (when not determined by chipping) was slightly       higher for wet blasted inserts                                                **)note: chipping observed in ascoated inserts                                ***)note: chipping observed in blasted inserts                           

                  TABLE 5                                                         ______________________________________                                        Tool Life and Failure Mode, Cast Iron SS0130                                  (Wet Blasted)                                                                 ______________________________________                                        Turning:             SS0130                                                   Cutting Speed:       300 m/min                                                Feed:                0.25 mm/r                                                Depth of Cut: 2.5 mm                                                          Insert Style:        CNMG 120408-M3                                           Coolant:             no                                                       ______________________________________                                        Coating    Failure Mode                                                                             Average Lifetime/min                                    ______________________________________                                        No. 1      flank wear 25                                                                 (uniform)                                                          No. 2      flank wear 25                                                                 (uniform)                                                          No. 3      flank wear 22                                                                 (uniform)                                                          No. 4      flank wear*)                                                                             11                                                                 (localized)                                                        No. 5      flank wear*)                                                                             10                                                                 (localized)                                                        ______________________________________                                         *)note: flaking of the coating and chipping are observed                 

                  TABLE 6                                                         ______________________________________                                        Edge Strength/Chipping Resistance, Steel SS1672                               ______________________________________                                        Turning against shoulder:                                                                         SS1672                                                    Cutting Speed:      200 m/min.sup.-1                                          Feed:               0.4 mm/r.sup.-1                                           Depth of Cut:       2.0 mm                                                    Insert Style:       CNMG 120408-M3                                            Coolant:            no                                                        ______________________________________                                                Number of Passes                                                      Coating       as-coated                                                                              wet blasted*)                                          ______________________________________                                        No. 1         10        50+                                                   No. 2         5         50+                                                   No. 3         5        40                                                     No. 4         5        15                                                     No. 5         5        10                                                     ______________________________________                                         *)note: SEM was used to select inserts exhibiting no TiN flaking at the       cutting edges for the test                                               

                  TABLE 7                                                         ______________________________________                                        Tool Life, Steel SS1672                                                       (Wet Blasted)                                                                 ______________________________________                                        Turning:          SS1672                                                      Cutting Speed:    250, 310, 375 m/min                                         Feed:             0.25 mm/r                                                   Depth of Cut:     2.5 mm                                                      Insert Style:     CNMG 120408-M3                                              Coolant:          no                                                          ______________________________________                                        Coating Structure                                                                        Cutting Speed                                                                             Average Lifetime                                                                           Δt/%                                ______________________________________                                        No. 6      375 m/min   3 min        --                                        No. 7      375 m/min   5 min*)      +67%                                      No. 3      375 m/min   9 min*)      +200%                                     No. 6      310 m/min   5 min        --                                        No. 7      310 m/min   7 min*)      +40%                                      No. 3      310 m/min   11 min*      +120%                                     No. 6      250 m/min   18 min       --                                        No. 7      250 m/min   20 min       +11%                                      No. 3      250 m/min   22 min       +22%                                      ______________________________________                                         *)note: alumina coated inserts showed less plastic deformation at higher      cutting speeds                                                           

                  TABLE 8                                                         ______________________________________                                        Blasting Response                                                             Coating  Toughness     Flaking    Production                                  Structure                                                                              (number of passes)*)                                                                        Resistance Yield                                       ______________________________________                                        No. 3    40            --         80% flaked                                  No. 8    40            --         45% flaked                                  No. 9    40            +          20% flaked                                  No. 10   40            ++         <2% flaked                                  ______________________________________                                         *)note: alumina coated inserts showed less plastic deformation at higher      cutting speeds                                                           

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A coated cemented carbide body comprising asubstrate of a cemented carbide, a layer of Al₂ O₃ and an outer layer ofwet blasted TiN at least 4 μm in thickness, the outer layer comprising amultilayer of TiN/TiC.
 2. The coated cemented carbide body of claim 1wherein the TiN/TiC multilayer is from 4 to 15 μm and comprises 5 to 20layers.
 3. The coated cemented carbide body of claim 1 wherein the Al₂O₃ is α-Al₂ O₃.
 4. The coated cemented carbide body of claim 3 whereinthere is a bonding layer between the α-Al₂ O₃ and the outer layer. 5.The coated cemented carbide body of claim 1 wherein the layer of Al₂ O₃comprises a layer of α-Al₂ O₃, a bonding layer and a layer of κ-Al₂ O₃.6. The coated cemented carbide body of claim 5 wherein the bonding layercomprises a layer of (Ti,Al)(C,O,N) deposited on said layer of α-Al₂ O₃and a layer of TiC deposited on said (Ti,Al)(C,O,N) layer.
 7. The coatedcemented carbide body of claim 1 wherein the Al₂ O₃ is thicker >8 μm. 8.The coated cemented carbide body of claim 1 wherein the TiN outer layeris from 4 to 15 μm.
 9. The coated cemented carbide body of claim 1wherein the outer TiN layer is greater than 4 μm.
 10. A method ofcutting Ca-treated steels using a cemented carbide cutting insert, theimprovement comprising using the cutting insert of claim 1.